when DotNetNuke vaguely says: "An error has occurred."

Look in the EventLog table of the DotNetNuke database for the most recent details on what is going wrong like so:

SELECT LogTypeKey, LogProperties, LogCreateDate FROM EventLog ORDER BY LogCreateDate DESC

 
 

You will want to copy the XML contents of the LogProperties column out to Notepad++ and look at it. You should see some message as to what is going on in there. If you use the LogCreateDate column to look at the time you may notice that several entries happen back to back and in these cases the topmost entry may not be the most important. You may want to look down the list a bit, but just a bit. LogTypeKey denotes different categories of entries.